In Fourth-Generation (4G) mobile communication systems, cells are configured to have very small radiuses in order to enable high-speed communications and accommodate a larger number of calls. Centralized network design is not viable for the 4G mobile communication systems. Rather, a wireless network should allow for distributed control and implementation and cope actively with an environment change, such as addition of a new Base Station (BS). That is why the 4G mobile communication systems require a self-configurable wireless network that is automatically or distributedly configurable without control of a centralized system.
For real deployment of the self-configurable network, technologies used for an Ad Hoc network are introduced to the 4G communication systems. That is, a multi-hop relay scheme used for the Ad Hoc network is adopted for a wireless network with fixed base stations.
Since communications are conducted between a fixed BS and a Mobile Station (MS) usually via a direct link, a highly reliable radio communication link can be easily established between them in a wireless communication system. However, the fixedness of base stations impedes flexible wireless network configuration, which makes it difficult to provide efficient services in a radio environment experiencing a fluctuating traffic distribution and a great change in the number of required calls.
To avert this problem, a relay scheme is adopted in which data is conveyed through multiple hops via neighbor mobile stations or neighbor relay stations. The multi-hop relay scheme facilitates fast network reconfiguration adaptive to an environmental change and renders the overall wireless network operation efficient. Also, a radio channel with better quality can be provided to a mobile station by installing a relay station between the base station and the mobile station and thus establishing a multi-hop relay path via the relay station. What is better, high-speed data channels can be provided to mobile stations in an area where communications with the base station are unavailable, and cell coverage is also expanded.
FIG. 1 illustrates the configuration of a typical wireless relay communication system.
Referring to FIG. 1, a mobile station 110 within the service area 101 of a base station (BS) 100 communicates directly with the base station 100. On the other hand, a mobile station 120, which is located outside the service area 101 of the base station 100 and thus placed in a poor channel status, communicates with the base station 100 via a relay station (RS) 130.
Through the RS 130, the base station 100 can communicate with mobile stations that are located in a shadowing area, which has severe shielding effects due to buildings, for example, and thus, which offers a poor channel status.
FIG. 2 illustrates a frame structure for a conventional wireless relay communication system.
Referring to FIG. 2, a frame is divided into a downlink sub-frame 200 and an uplink sub-frame 230.
The downlink sub-frame 200 includes a first zone 210 in which a base station provides a service via a direct link and a second zone 220 in which a relay station provides a service via a relay link.
Accordingly, the base station configures a BS downlink subframe to be transmitted to a relay station or a mobile station connected to the base station via a direct link in the first zone 210. The BS downlink subframe is composed of a preamble 211, a control channel 213, and downlink bursts 215.
The relay station configures an RS downlink subframe to be transmitted to a lower relay station or a mobile station connected to the relay station via a relay link in the second zone 220. The RS downlink subframe is composed of a preamble 221, a control channel 223, and downlink bursts 225.
The uplink sub-frame 230 includes a first zone 231 for direct-link communications with the base station and a second zone 233 for relay-link with the relay station.
Accordingly, a relay station or a mobile station connected to the base station via a direct link configures a BS uplink subframe in the first zone 231 in order to transmit control information and traffic to the base station. An MS connected to a relay station via a relay link configures an RS uplink subframe in the second zone 233 in order to transmit control information and traffic to the relay station.
A guard region called a Transmit/receive Transition Gap (TTG) 240 is interposed between the downlink subframe 200 and the uplink subframe 230, and a guard region called a Receive/transmit Transition Gap (RTG) 250 is interposed between frames.
This frame structure brings different frame timings to mobile stations depending on what entity (e.g., BS or RS) provides services to them. For example, when a base station serves a mobile station, the mobile station receives a service in the BS downlink subframe in the first zone 210. If a relay station serves the mobile station, the mobile station receives a service in the RS downlink subframe in the second zone 220.
As described above, if mobile stations have different frame timings according to an entity that serves them, handover and synchronization are difficult to achieve.